1. Field of the Invention
This invention relates to a pickup tone arm of a record player and particularly to a pickup tone arm which can simultaneously improve a low frequency range resonance (hereinafter referred to as a low resonance) f.sub.l caused by the compliance of a stylus of a pickup cartridge and the equivalent mass of a tone arm and a middle frequency range resonance (hereinafter referred to as a middle resonance) f.sub.m of the tone arm itself thereby preventing the bend down of a complex resonance type pickup tone arm which is divided in plural sections with a damping member being disposed in the tone arm.
2. Description of the Prior Art
Recently, for enhancing the compliance of a pickup cartridge, the low resonance f.sub.l of a pipe-type tone arm has been a lower frequency range such as 4 to 12 H.sub.z and the peak level at its resonance frequency has been quite high, for example, 10 to 15 dB as compared with the normal level. In addition, a record disc itself has eccentricity, warp and so on, so that when the record disc is reproduced, wow is caused whose frequency is rather high as compared with the resonance frequency f.sub.l. It is also known that the lower frequency limit of an audio frequency band at its reproduction can be selected as about 20 H.sub.z. Nowadays, therefore, as the compliance of the pickup cartridge becomes higher the low resonance f.sub.l is designed to be selected not in a range of 4 to 12 H.sub.z but at a frequency lower than or equal to 20 H.sub.z.
Now, in order to increase the compliance of a tone arm, consideration will be given to the case where a record disc having warp equivalent to, for example, more than 6 H.sub.z is faithfully traced by means of a pickup cartridge tone arm having a low resonance of about 6 H.sub.z. In this case, the tone arm can not follow the record disc having such a warp since the frequency band higher than 6 H.sub.z becomes difficult to follow. so that the vibration system of a cantilever including a stylus of the pickup cartridge follows the warp of the record disc equivalent to 6 H.sub.z to which deteriorates its sound quality, because it contains a warp component.
As a condition of increasing the compliance of the pickup cartridge, the equivalent mass of the tone arm viewing from its stylus is made small. However, when the tone arm is designed light in weight for the above purpose, it is limited naturally from a view point of mechanical strength of the tone arm itself. Further, even though the low resonance is selected in a range between 15 and 20 H.sub.z outside the frequency band where there are a lot of scattered frequency components of warp, eccentricity or the like of the record disc, the resonance peak is still present, so that a low frequency component in the vicinity thereof is included in a musical sound of a record source or in its envelope. When the tone arm receives an external vibration energy of the same frequency as that of low frequency components in the musical sound, it vibrates resonance. The higher the value of Q, the stronger becomes the vibration and hence the stylus of the pickup cartridge is moved substantially. Thus, a frequency component higher than f.sub.l will be modulated so that an effect which deteriorates the quality of reproduced sound is further increased. As a result, if a condition where the resonance peak is not damped as shown by a curve (a) in frequency-response characteristics of FIG. 1 can be changed to a condition where the same is subjected to critical damping as shown by a curve (b) and the low resonance f.sub.l or a so-called cut-off frequency can be established at an optimum position, the problems as described above will be solved.
In the prior art, as a means for damping the low resonant frequency a device utilizes viscous resistance such as oil or the like in its bearing portion. In this case, however, since the reproducing band extends to a frequency lower than the cut-off frequency or lower than 5 H.sub.z in FIG. 1 and there occurs a problem of compliance ability for warp, eccentricity or the like of the record disc, the pickup cartridge vibrating system receives DC displacement and sometimes with a biasing force much exceeding the inside force at the record play time, so that the quality of the reproduced sound from the record disc may be badly affected.
In order to achieve the same purpose as described above, there has been proposed a damper device utilizing anti-resonance as shown in FIG. 2, wherein a balance weight 2 is attached through a resilient coupler 3 to the rear end of a tone arm 1 which is pivotally mounted on a base 6. In this device, if the complicance of the pickup cartridge is not properly selected, its frequency-response characteristic is shown by a dotted-line curve (4) in FIG. 3 wherein the response of one peak resonance frequency becomes higher than the other and it is very difficult to obtain responses at two peak resonance frequencies which are equal as shown by a full-line curve (5) in FIG. 3. Therefore, there are drawbacks that the functional property of a universal tone arm is absent and mechanical vibrations from the base 6 are greatly applied to the arm.
Further, the prior art tone arm has another problem of the so-called middle resonance according to the resonance of a pipe, other mechanisms and the like. It has been found that the greatest middle resonance exists normally in a range between 50 and 200 H.sub.z and many other middle resonances are scattered over the wide range according to the difference of structures or the like.
In a normal device for measuring the middle resonance by using a frequency characteristics measuring record disc, a damper of high compliance is coupled between the pickup cartridge stylus and the tone arm, that is, a mechanical filter of very low cut-off frequency is considered to be interposed therebetween in this case, so that the vibrating energy applied to the tone arm is not enough to excite it and hence the middle resonance does not appear substantially in the frequency characteristics. However, when the tone arm 1 is driven from the side of the base 6, the middle response is greatly affected.
In FIG. 4, an exciter device 7 is arranged at the side of the base 6 of the tone arm 1. This exciter device 7 is energized by an oscillating frequency of an oscillator 8 and a signal from a detector 9 is applied to a recorder 10 for recording so that the frequency-response characteristics are obtained as shown in FIG. 5 by a curve (11). Accordingly, when the tone arm is assembled as a unit of the record player, upon playing a record a sound energy from a loudspeaker is fed back through a player cabinet from the base 6 to the tone arm 1 to excite the tone arm main holder or directly the tone arm 1 itself. Therefore, the tone arm 1 becomes resonant to deteriorate the reproduced sound quality, and when the record player or the like is touched with by hand, noise or howl is produced. One of the great causes for producing the above noise or howl is this effects the middle resonance.
One embodiment for providing a tone arm capable of improving the low and middle resonances simultaneously will next be described with reference to FIG. 6. In FIG. 6, reference numeral 12 designates a stylus of the pickup cartridge and 13 a pipe portion of an aluminium arm. The end portion of the pipe 13 is fixed to a cylindrical holder 14 and one end 14a of the holder 14 is connected to one end of a suspension wire 15 made of a piano wire, high polymer material or the like passing through the center of the holder 14.
The other end of the suspension wire 15 is fixed to one end of a pipe holder 18 which is inserted in a bore 17 provided at the center of a tone arm main holder 16. Between the holder 14 and the tone arm main holder 16 is interposed a damper member 19 made of visco-elastic material or the like. After being adjusted at an optimum position, the pipe holder 18 is secured thereto by adhesive material, a set screw 20 or the like.
With the construction of the tone arm as described above, the tone arm main holder 16 and the pickup cartridge pipe 13 are isolated from each other in a vibratory manner with the damper 19 being interposed therebetween so that the tone arm may have the same elasticity and viscosity in every direction over an angular range of 360.degree. in a universal joint manner with the position of the suspension wire 15 inserted in the center of the damper 19 being set as a fulcrum. In this case, however, the tone arm will not move in a front-rear direction, that is, the axial direction of the pipe 13 by means of the suspension wire 15. In this construction, it is noticed preferable that, as shown in FIG. 6, the damper 19 is disposed at the pickup cartridge side of the tone arm within substantially 50 mm of the distance L from the shaft 21 of the base 6 to form the complex resonance system vibration fulcrum.
A description will next be given of an operation theory of the above-mentioned construction. The above construction can be illustrated by an electrical equivalent circuit as shown in FIG. 7. In the equivalent circuit of FIG. 7, s.sub.r and r.sub.r denote respectively an equivalent stiffness and equivalent resistance which show elasticity and loss of the record disc, and m.sub.c, s.sub.c and r.sub.c represent respectively an equivalent mass of the pickup cartridge vibration system and an equivalent stiffness and equivalent resistance of the damper of the vibration system holding portion. Further, m.sub.t1 designates an equivalent mass of the complex resonance system vibration fulcrum including the pickup cartridge, that is, of the pickup cartridge side portion from the damper 19, s.sub.t and r.sub.t respectively an equivalent stiffness and equivalent resistance of the complex resonance system fulcrum and m.sub.t2 and m.sub.t3 respectively equivalent masses of the portions of the tone arm main holder 16 excepting the equivalent mass m.sub.t1, r.sub.p an equivalent resistance of the pivotal portion of the tone arm fulcrum, v a sound groove speed amplitude of the record disc and z.sub.m a mechanical impedance looking from the record stylus, respectively.
In the proposed low frequency sound range, m.sub.c, s.sub.r and r.sub.r can be considered negligible, so that the equivalent circuit of FIG. 7 can be approximated to an equivalent circuit of FIG. 8. Assumed that relations s.sub.c &lt; s.sub.t and r.sub.c &lt; r.sub.t are satisfied in the equivalent circuit of FIG. 8, a frequency f.sub.1 of a resonance circuit consisting of m.sub.t1, s.sub.c and s.sub.t and a frequency f.sub.2 of a resonance circuit consisting of s.sub.c, m.sub.t1 and m.sub.t2.3 are established with a condition of f.sub.1 &gt; f.sub.2. The frequency-response characteristics for the above is shown by a curve (23) in FIG. 9 and the low frequency characteristics of the prior art tone arm is shown by a curve (24) in FIG. 9.
According to the tone arm as mentioned above, there is an advantage that the complex resonance frequencies f.sub.1 and f.sub.2, that is, the low resonance f.sub.l can keep the value of Q low and the level at the lower resonance frequency f.sub.2 can be desirably kept low. Further, the effect for the above tone arm is substantially independent of the amount of compliance of the pickup cartridge and the optimum value thereof can also be selected according to the value of the compliance. In addition, there is used a method of adjusting its damping function by changing the aforesaid s.sub.t and r.sub.t of the complex resonance system fulcrum. Similarly in the case of the middle resonance f.sub.m, a resistance component is inserted between the stylus and the tone arm rear portion thereby to damp the resonance produced therebetween. Particularly, when an external vibration is transmitted from the arm base, the above tone arm is quite effective, and in the case of record reproduction it is also effective for the prevention of howling or the like.
As frequently seen in a prior art tone arm as shown in FIG. 10, where a visco-elastic material 25 is disposed at the side of the rear weight portion, the vibration frequency characteristic thereof is shown in FIG. 5 by a curve (26). As compared with the above described tone arm with its characteristic curve (27), a big difference was found therebetween. A pickup device with a visco-elastic material 28 being inserted in the middle of a tone arm pipe as shown in FIG. 11 has already been disclosed in The AES, 1963, 6 Vol. 11 No. 3 by Mr. B. B. Bauer of USA. However, in the case of high compliance such as the recent pickup cartridge, if the compliance of the visco-elastic material inserted in the middle of the pipe or the like is not increased, no effect appears resulting in poor utility.
Another embodiment of this construction will be described in detail with reference to FIG. 12 in which elements corresponding to those of FIG. 6 are shown by the same reference numerals with their description being omitted. This pickup cartridge device is provided with a damping adjustable mechanism or the like which can adjust the damping function in conformity with the compliance of the pickup cartridge. That is, the pipe holder 14 is provided at its center with a bore 14c into which the tone arm pipe 13 is inserted and secured to the holder 14 by a set screw 33. The suspension wire 15 provided in the central bore of the damper 19 is fixed at its one end to a first suspension holder 14b and at its other end to a second suspension holder 18 which is inserted into the bore 17 of the tone arm main holder 16. A lead wire 34 from the cartridge is passed through the central aperture of the damper 19 for stretching the pipe 13 and penetrates through the base 6. Also, a balancing weight 29 is disposed on the tone arm main holder 16 in an axially slidable manner. The main holder 16 has bored therein a facing aperture 30 into which is inserted a damping control rotary knob 32 formed substantially in a screw type. The damping control rotary knob 32 is provided at its top end 32a with an internal thread 32b to which is engaged a screw formed about the top end of the second suspension wire 18. Thus, the damping control rotary knob 32 is connected to the suspension wire 15, the pipe 13 and the like. In FIG. 12, 31 designates a rotary knob stopper. With the construction as mentioned above, the damping can be adjusted by rotating the knob 32 which is quite effective for the low and middle resonances f.sub.l and f.sub.m with the result that the sound quality is greatly improved upon reproducing a record disc.
In the respective constructions as described above, since the suspension wire 15 is stretched between the pipe 13 and the balance weight (the main holder portion) 16 at their axial center, the damper 19 is applied with an objective pressure to maintain the tone arm pipe 13 and main holder 16 so as not to be bent or hang down. However, unless the visco-elestic material to be used is relatively hard, it is bent or hangs down as mentioned above. For this reason, the suspension wire 15 is usually applied with a relatively large tension. However, such an arrangement will tend to lessen the effect in that the visco-elastic material is interposed to provide a filter effect.